Vertical transmission line structure that includes bump elements for flip-chip mounting

ABSTRACT

A vertical transmission structure for high frequency transmission lines includes a conductive axial core and a conductive structure surrounding the conductive axial core. The vertical transmission structure is applied to a high-frequency flip chip package for reducing the possibility of underfill from coming in contact with the conductive axial core.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 12/196,529,filed on 22 Aug., 2008, now U.S. Pat. No. 7,940,143, issued May 10,2011, and for which priority is claimed under 35 U.S.C. §120; and thisapplication claims priority of Application No. 97115188 filed in Taiwanon 25 Apr. 2008 under 35 U.S.C. §119; the entire contents of all ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vertical transmission structures, andvertical transmission structures for high-frequency transmission lines.

2. Background of the Related Art

Along with the rapid advances in wireless communication productsdevelopment in recent years, a planar PCB architecture can not satisfythe demand of low cost, high density and compact products. Thus, thevertical interconnection of low temperature co-fired ceramics (LTCC)package or multi-layer PCB have gradually replaced the design of theplanar PCB. However, the via design for the multi-layer PCB or LTTC maycause parasitic impedances or inductances.

There have been various types for vertical transmission, such astransmission between microstrip line and strip line, microstrips,coplanar waveguides, or coplanar waveguide and strip line. Taking thefirst type as an example, the vertical transmission interconnectsbetween the coplanar waveguides do not cause large return loss in lowfrequencies, however, the parasitic effects from the verticaltransmission interconnects can degrade the characteristics of returnparameters with increase in the operating frequencies. Generally, thecompensation of local matching may reduce the parasitic effects andfurther convert them to an inductance effect to achieve both theimpedance match and prevent the parasitic effects. In the second type,the microstrip line transmission with slots or cavities does not adopt avia architecture to prevent the parasitic effects. The slotconfiguration may generate inductance effects to achieve impedancematch. The improved cavity configuration includes both a dielectriclayer between two ground layers made of a same metallic material and aslot to perform coupling by waveguides. In the third type, thetransmission between microstrip line and strip line improves frequencyband characteristics by high impedance compensation that utilizes ahigh-impedance line with additional inductance to compensate thecapacitance of transmission lines. It may be due to different impedancesgenerated by the lines with various widths. The impedances may be raisedby decreasing the widths and converted into inductances.

FIG. 1 is a top-view perspective diagram illustrating a conventionalthree ground bump configuration of a high-frequency flip chip package. Asubstrate 10 has a coplanar waveguide 12. The circuit layer 16 of amicrowave chip package 18 is coupled to the coplanar waveguide 12 viathree bumps 14. There are various transmission structures applied tovarious packages. Basically, the parasitic effects from various wiringconfigurations of the packages are raised with increase of operatingfrequencies. Thus, flip chip package has been gradually applied tohigh-frequency products. However, when the underfill comes in contactwith the transmission structures of the flip chip package, significantwaveband frequency loss of transmission lines occurs. Attempts are stillbeing made to improve the transmission architecture of transmissionlines to achieve reduction in the loss of waveband frequency.

SUMMARY OF THE INVENTION

The present invention is directed to a transmission structure, which maybe suitable for reducing the possibility of the underfill from coming incontact with the vertical transmission structure between connected linesto increase the performance of the transmission lines.

The present invention is also directed to a transmission structure whichmay be suitable for improving the transmission efficiency between amicrowave chip and a package substrate and reduce crosstalk, insertionloss and return loss of the signal lines.

The present invention is also directed to a transmission structurehaving a coaxial configuration suitable to provide more return currentpaths.

Accordingly, one embodiment of the transmission structure of a chipincludes a chip having an insulating surface; a signal line disposedover the insulating surface, wherein the signal line includes a bar bumpand two terminals; a ground bump separately disposed over the insulatingsurface, set coplanarly with and electrically isolated from the signalline, and an expansion bump formed around each terminal; a dielectriclayer disposed over the signal line and the ground bump, wherein thedielectric layer is in solid form and exposes the two terminals and aportion of each expansion bump; two first conductive connectors disposedover the dielectric layer, wherein the two first conductive connectorsrespectively correspond to and in contact with the two exposed expansionbumps; and two second conductive connectors disposed over the dielectriclayer and respectively in contact with the two exposed terminals,wherein each of the second conductive connectors respectively serves asa center of axis of the first conductive connectors.

Another embodiment of the transmission structure of a high-frequencytransmission line includes a substrate having an insulating surfaceformed thereon; a pattern of high-frequency transmission structuresdisposed over the insulating surface, wherein the pattern ofhigh-frequency transmission structures includes a signal line having abar bump and a terminal; and a ground bump separately set over thesubstrate, set coplanarly with and electrically isolated from the signalline, wherein the ground bump surrounds the bar bump and includes anexpansion bump surrounding the terminal; a dielectric layer disposedover the pattern of high-frequency transmission structures, wherein thedielectric layer is in solid form and exposes the terminal and theexpansion bump of the ground bump; a first conductive connector disposedover the dielectric layer and in contact with the exposed expansionbump; and a second conductive connector disposed over the dielectriclayer and in contact with the exposed terminal, wherein the secondconductive connector serves as a center of axis of the first conductiveconnector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top-view perspective diagram illustrating a conventionalthree-grounding bump of a high-frequency flip chip package;

FIGS. 2A to 2E are cross-sectional perspective diagrams illustrating aprocess of fabricating a substrates of a high-frequency flip chippackage according to one embodiment of the present invention;

FIG. 3 is a top-view perspective diagram illustrating a high-frequencyflip chip package according to one embodiment of the present invention;

FIG. 4A is a schematic diagram illustrating the combination of asubstrate and package according to one embodiment of the presentinvention; and

FIG. 4B is a scaled-up schematic diagram illustrating a transmissionstructure according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2A to 2E are cross-sectional perspective diagrams illustrating aprocess of fabricating a substrates of a high-frequency flip chippackage according to one embodiment of the present invention. As shownin FIG. 2A, a conductive layer is formed over a substrate 11 having aninsulating surface. A pattern of high-frequency transmission structuresis transferred onto the conductive layer. The pattern of high-frequencytransmission structures includes a signal line 15 and a ground bump 13set coplanarly with and electrically isolated from the signal line 15.In one embodiment, the substrate 11 may be comprised of a single ormultiple layers made of glass, silicon, ceramic or polymer material. Theconductive layer may include a copper foil laminated with the substrate11, a copper foil with electroplated copper, or a substrateelectroplated with an electroplated copper. Alternatively, theconductive layer may include, for example but not limited to, a goldlayer, and may be formed over the substrate 11 by lithography andelectroplating processes.

Next, the signal line 15 extends from one side of the substrate 11 tothe center thereof, and comprises a bar bump and a terminal 151. Theground bump 13 extends from the side of the substrate 11, and includes akey hole shaped opening including an expansion bump 131 surrounding thebar bump of the signal line 15 such that the space between the groundbump 13 and the bar bump of the signal line 15 is smaller than thatbetween the expansion bump 131 of the ground bump 13 and the terminal151 of the signal line 15. In the present embodiment, the expansion bump131 has an arc shape. Alternatively, the expansion bump 131 may alsohave other geometric shapes, such as but not limited to a square or adiamond shape.

Referring to FIG. 2B, a dielectric layer 17 is formed over the patternof high-frequency transmission structures and a portion of the substrate11. Next, a portion of the dielectric layer 17 in solid form is removedby using any well known process, such as etching, to expose the terminal151 and the expansion bump 131. In one embodiment, the dielectric layer17 may be made of benzocyclobutene, photo-benzocyclobutene, polyimide,nitride, oxide or ceramic material.

As shown in FIG. 2C, a photoresist layer 19 is formed over thedielectric layer 17 and the substrate 11 by using any well knownprocess, such as spin coating or thy film covering process. Thephotoresist layer 19 is subjected to photolithography and etchingprocess to form a first pattern 191 and a second pattern 193. Theterminal 151 is exposed by the second pattern 193. The expansion bump131 and a portion of the dielectric layer 17 are exposed by the firstpattern 191. The exposed portion of the expansion bump 131 and theexposed portion of the dielectric layer 17 constitute a coaxial circularpattern with the terminal 151. It is understood that the shape of theexpansion bump 131 and the terminal 151 is not limited to be a circularpattern; the expansion bump 131 may also have other coaxial geometricshapes.

Next, an electroplating or an evaporation process is carried out to forma conductive layer over the first pattern 191 and the second pattern193, such as either a titanium layer or a gold layer thereon. The wholestructure is first subjected to a curing process and then thephotoresist layer 19 (shown in FIG. 2C) is removed to form a firstconductive connector 201 and a second conductive connector 203protruding out from the dielectric layer 17 over the substrate 11, asshown in FIG. 2D. The first conductive connector 201 is electricallyconnected to the ground bump 13 and electrically isolated from thesignal line 15. The second conductive connector 203 is electricallyconnected to the terminal of the signal line 15 and electricallyisolated from the ground bump 13. The first conductive connector 201 hasa fully enclosed shape, such as but not limited to a fully enclosed ringshape. The second conductive connector 203 has a rod shape and serves asa center of axis of the first conductive connector 201. Shown in FIG. 2Eis a substrate structure for a high-frequency flip chip package formedwith two of the structures shown in FIG. 2D disposed back-to-back. Thesubstrate structure for high-frequency flip chip package includes asignal line 15 divided into two parts, a ground bump 13 divided in totwo parts, two first conductive connectors 201 a and 201 b respectivelyconnecting with the two parts of the ground bump 13 and two secondconductive connectors 203 a and 203 b respectively connecting with thetwo parts of the signal line 15. The first and second conductiveconnectors 201 a, 201 b and 203 a, 203 b may be located over a centralarea of the substrate 11. Moreover, the signal line 15 and ground bump13 located over the edge of the substrate 11 is exposed for use forconnection.

FIG. 3 is a top-view perspective diagram illustrating a high-frequencyflip chip package according to one embodiment of the present invention.As shown in FIG. 3, the active surface (not shown in this figure) of achip 29 is faced down. A dielectric layer 27 is disposed on the circuitside of the chip 29. The circuit side has two of the patterns ofhigh-frequency transmission structures similar to one shown in FIG. 2Adisposed face-to-face, which includes a signal line 231 and a groundbump 23 around the signal line 231. First conductive connectors 251 aand 251 b and second conductive connectors 253 a and 253 b are formed byusing the same process steps illustrated with reference to FIG. 2B toFIG. 2E. The first conductive connectors 251 a and 251 b respectivelycorrespond to and disposed around the second conductive connectors 253 aand 253 b. The first conductive connectors 251 a and 251 b areelectrically connected to the ground bump 23. One end of the secondconductive connectors 253 a and 253 b respectively is electricallyconnected to the signal line 231.

FIG. 4A is a schematic diagram illustrating the combination of asubstrate and a flip chip package according one embodiment of thepresent invention. Referring to FIG. 4A, FIG. 2A and FIG. 2E, thesubstrate 11 includes a first signal line 15 and a first ground bump 13coplanarly and separately located and electrically isolated from eachother, wherein the first signal line 15 includes two first bar bumps andtwo first terminals 151 (one shown in FIG. 2A), and the first groundbump 13 includes two first expansion bumps 131 each surrounding one ofthe first terminals 151 (one shown in FIG. 2A). Referring to FIG. 4A,the dielectric layer 17 covers a portion of the first ground bump 13 anda portion of the first signal line 15. The flip chip 29 includes asecond signal line 231 and a second ground bump 23 that are coplanarlyand separately located and electrically isolated from each other,wherein the second signal line 231 includes a second bar bump and twosecond terminals 2311, and the second ground bump 23 includes two secondexpansion bumps 233 each surrounding one of the second terminals 2311.

FIG. 4B is a scaled-up schematic diagram illustrating a transmissionstructure according to one embodiment of the present invention. Thedielectric layers 17, 27 (see FIG. 4A) are omitted from the drawing forthe convenience of illustration. As shown in FIG. 4B, for forming thetransmission structure, the first signal line 15 over the substrate 11is connected to the second signal line 231 of the flip chip via thestructural connection of the second conductive connector 203 a/203 bshown in FIG. 2D and the second conductive connector 253 a/253 b shownin FIG. 3 and FIG. 4B. The first ground bump 13 over the substrate 11 isconnected to the second ground bump 23 of the flip chip via thestructural connection of the first conductive connector 201 a/201 b inFIG. 2D and the first conductive connector 251 a/251 b shown in FIG. 3and FIG. 4B.

Referring to FIG. 4A and FIG. 4B, in other words, the first conductiveconnector (the connected first conductive connectors 201 a/201 b and 251a/251 b) is located between the substrate and the flip chip, andelectrically connected to the first expansion bump 131 (see FIG. 2A) ofthe first ground bump 13 and the second expansion bump 233 of the secondground bump 23 (see FIG. 4A). The first conductive connector has a fullyenclosed shape, such as but limited to a fully enclosed ring shape. Thesecond conductive connector (the connected second conductive connectors203 a/203 b and 253 a/253 b) is located between the substrate and theflip chip, and structurally in contact with the first terminal 151 (seeFIG. 2A) of the first signal line 15 and the second terminal 2311 (seeFIG. 4A) of the second signal line 231. The second conductive connectorhas a rod shape and serves as a center of axis of the first conductiveconnector.

Accordingly, the application of the transmission structure to ahigh-frequency flip chip package may reduce the possibility of underfillfrom causing a vertical transmission between connected lines. Thus, thetransmission structure may enhance the transmission efficiency of amicrochip and a package substrate and further reduce both insertion loss(smaller than 0.6 dB) and return loss (smaller than 20 dB).

The vertical transmission structure may be applied, for example but notlimited to, the transmission between microstrip line and strip line,microstrip lines, coplanar waveguides, or coplanar waveguide andmicrostrip line. Moreover, the vertical transmission structure may beapplied to the flip chip package, low temperature co-fired ceramicspackage (LTCC), high temperature co-fired ceramics package (HTCC),organic laminate multichip modules (MCM-L) or deposited thin film MCM(MCM-D).

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

1. A transmission structure of a high-frequency transmission line,comprising: a substrate, comprising an insulating surface; a pattern ofhigh-frequency transmission structures, formed over the insulatingsurface, comprising: a signal line, having a bar bump and a terminal;and a ground bump, set coplanarly with and electrically isolated fromthe signal line, and comprising an expansion bump surrounding theterminal; a dielectric layer, formed over the pattern of high-frequencytransmission structures, wherein the dielectric layer is in solid formand exposes the terminal and the expansion bump of the ground bump; afirst conductive connector, disposed over the dielectric layer and incontact with the exposed expansion bump; and a second conductiveconnector, disposed over the dielectric layer and in contact with theexposed terminal, wherein the second conductive connector serves as acenter of axis of the first conductive connector.
 2. The transmissionstructure of a high-frequency transmission line according to claim 1,wherein the substrate comprises glass, silicon, ceramic or polymer. 3.The transmission structure of a high-frequency transmission lineaccording to claim 1, wherein the pattern of high-frequency transmissionstructures comprises a copper layer or a gold layer.
 4. The transmissionstructure of a high-frequency transmission line according to claim 1,wherein the first conductive connector and the second conductiveconnector comprise a titanium layer or a gold layer.
 5. The transmissionstructure of a high-frequency transmission line according to claim 1,wherein the first conductive connector comprises a rod shape and thesecond conductive connector comprises a fully enclosed shape surroundingthe first conductive connector.